It is well-known to convert a synthesis gas mixture as mentioned catalytically into methanol according to the reaction EQU CO+2 H.sub.2 .revreaction.CH.sub.3 OH (1)
Examples of suitable methanol synthesis catalysts are oxides of zinc and chromium; oxides of zinc, copper and chromium; and oxides of zinc, copper and aluminum; as well as zinc-copper-chromium-lanthanum oxides. Under typical conditions of reaction these catalysts will also catalyze the water gas reaction (shift reaction) EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 (3)
In conventional methanol synthesis, which typically takes place at 220.degree.-400.degree. C. and 10-500 kg/cm.sup.2, the degree of conversion per passage is usually low because of an unfavorable equilibrium of methanol formation in reaction (1),and for this reason high ratios of recycling are needed, usually from 4:1 to 10:1. In order to improve the equilibrium it has been proposed (in connection with the conversion of synthesis gas into petrol (gasoline)) in U.S. Pat. specification No. 3,894,102 to combine the methanol synthesis with the catalytical conversion of a substantial proportion of the methanol formed into dimethyl ether according to the methanol dehydration reaction EQU 2 CH.sub.3 OH.revreaction.CH.sub.3 OCH.sub.3 +H.sub.2 O (2)
Many materials are known to catalyze this reaction, notably the so called acidic dehydration catalysts as for instance .gamma.-alumina (which is employed according to the above patent specification), silica, silica-alumina and crystalline aluminosilicates such as zeolites.
Reactions (1) to (3) are normally carried out heterogenically in gas phase over a solid, optionally supported catalyst.
It is well-known to carbonylate alkanols and ethers catalytically into carboxylic acids containing one carbon atom more than the starting material, and esters or anhydrides thereof, specifically to carbonylate methanol and dimethyl ether to form acetic acid, its anhydride or methyl acetate according to the reactions EQU CH.sub.3 OH+CO.fwdarw.CH.sub.3 COOH (4) EQU CH.sub.3 OCH.sub.3 +2CO+H.sub.2 O.fwdarw.2CH.sub.3 COOH (4a) EQU CH.sub.3 OCH.sub.3 +CO.fwdarw.CH.sub.3 COOCH.sub.3 (5) EQU CH.sub.3 OCH.sub.3 +2CO.fwdarw.(CH.sub.3 CO).sub.2 O (6)
and EQU CH.sub.3 COOCH.sub.3 +CO.fwdarw.(CH.sub.3 CO).sub.2 O (7)
These reactions have been carried out in liquid phase and in gas phase over a solid catalyst bed, and various catalysts have been proposed, especially noble metal catalysts. A few publications on these reactions are mentioned in the following.
In U.S. Pat. specification No. 3,689,533 there is mentioned a number of drawbacks in older carbonylation reactions (4) and (5), notably a low degree of conversion and many by-products, in part difficult to remove. To overcome these drawbacks this patent proposes to react methanol and dimethyl ether with carbon monoxide in gas phase at a temperature of 50.degree.-500.degree. C. and a CO partial pressure of 0.07 to 1050 kg/cm.sup.2 abs., preferably 0.7-50 kg/cm.sup.2 abs., in the presence of a supported rhodium catalyst promoted with iodine, bromine or compounds thereof. It appears from the specification that CO is added in a pure form but in certain cases steam may be present.
The slightly elder British patent specification No. 1,233,121 enumerates the same drawbacks in known carbonylation reaction as the above US patent specification and proposes a similar reaction as that, but conducted in liquid phase (homogenous catalysis), viz. the carbonylation of an alkyl compound containing n carbon atoms (n being an integer 6-20) in the form of an alcohol, a halide, an ester or an ether by the reaction at a temperature of at least 50.degree. C. with carbon monoxide in the presence of a catalyst of rhodium or a rhodium compound and a promoter selected amongst bromine, iodine and compounds thereof. In Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A1, page 47, VCH Verlagsgesellschaft, Weinheim, it is stated that this process, which is known as the Monsanto process when the alkyl compound is methanol and which has gained industrial spreading, leads to the carbonylation of methanol into acetic acid in a yield of 99%, calculated from methanol, and 90%, calculated from CO. Important by-products are stated to be hydrogen and carbon dioxide formed via the water gas shift reaction (3), which is known to be catalyzed by rhodium under homogenous process conditions (Ullmann, l.c. p. 49 and J. Am. Chem. Soc. 99, 2791, 1977), accompanied by a significant loss of valuable CO. In this process methanol is also added in pure form. The methanol process suffers, as mentioned, from unfavorable equilibria, which apart from a high reaction pressure call for cooling of the entire process stream, condensation of methanol and recycling of remaining large amounts of unconverted synthesis gas.
It is moreover known from U.S. Pat. specification No. 4,356,320 to conduct the carbonylation by the aid of a base metal catalyst, viz. a nickel catalyst in the presence of an organophosphorus(III) compound and using an iodide. Even in this case CO and methanol are added in pure form.
It is characteristic for the carbonylation processes mentioned that the carbonylation reaction is of the order zero with respect to carbon monoxide as well as methanol and dimethyl ether (Ullmann, l.c. page 48). As moreover the reactions (4)-(7) have equilibrium almost completely in favor of the products, one is free to choose space velocity, pressure and temperature in a way so as to obtain close to complete conversion of the reactants in one single passage through the reactor.
From German patent specification No. 26 10 036 there is known a process for preparing symmetric or unsymmetric carboxylic anhydrides of the formula (RCO).sub.2 O, wherein R denotes C.sub.1-4 alkyl and hence specifically acetic anhydride, from the corresponding alkyl alkanoates or alkyl ethers, i.e. specifically according to reactions (6) and (7), under substantially anhydrous conditions in the presence of noble metal catalysts of group VIII of the Periodical Table of Elements, and a complex promoter containing at least an organonitrogen or organophosphorus compound where N and P are trivalent, together with a metal belonging to groups IVa, Va or VIa of the Periodical Table of Elements; the reaction takes place at a CO partial pressure of 0.07-678 bar and a temperature of 25.degree.-350.degree. C. The specification mentions that carbonylation at low pressures earlier had only resulted in acetic acid, whereas carbonylation in acetic anhydride only could be carried out a very elevated pressures, a disadvantage which is told to have been remedied by using the abovementioned catalyst system. The reaction is normally carried out at moderately elevated pressure, e.g. 1-69 and preferably 2-14 bar, and the reaction time is in the range of 0.1-20 hours dependent of the temperature and pressure. It is advantageously conducted in liquid phase (homogeneously) with a solvent or diluent present. As noble metal catalyst there is preferably employed a rhodium compound.
German published patent specification No. 34 40 646 describes the preparation of monocarboxylic anhydrides (RCO).sub.2 O by the reaction of the corresponding alkyl alkanoates or alkyl ethers in gas phase by gas phase catalysis with a supported catalyst with an organosilicon compound having alkoxy or halogen groups as well as organo-nitrogen, organophosphorus, organoarsenic, organosulphur, mercapto or thioether groups as a polyfunctional attachment agent between the support at one hand and a noble metal compound of group VIII in the Periodical Table of Elements at the other hand. The specification says that there is obtained an improved catalyst activity and selectivity compared to the use of supports impregnated with catalyst solutions.
The fundamental reactions in the conversion according to the invention thus are well-known per se but it has not been suggested to combine them in one reaction sequence as here proposed. However, it has now been surprisingly found that there is obtained a technically simple reaction sequence by such a combination, and a very high degree of conversion already after one passage based on the synthesis gas, by such a combination. Under certain circumstances there is obtained practically full conversion of synthesis gas to acetic acid by one passage only.
As explained, in the known processes for carbonylation of methanol and dimethyl ether to acetic acid and methyl acetate or the anhydride, the needful carbon monoxide has been added as such. In practice carbon monoxide is normally obtained by various reactions which involve the reforming of methane or higher hydrocarbons. In the synthesis gas thereby formed, CO is present along with H.sub.2 and CO.sub.2 which accordingly must be separated, e.g. by an expensive cryogenic separation, in order to provide the feed gas needed. It is a particular advantage in the present process that one avoids this separation, avoids a separate preparation of methanol or dimethyl ether (or mixtures thereof), and in a coherent reaction sequence by a suitable control of the reactions and recycling streams can obtain a high conversion in just one passage of a synthesis gas as stated into acetic acid, methyl acetate, acetic anhydride or mixtures thereof.